In continuous casting, liquid metal is poured into the upper end of and withdrawn from the lower end of a vertically throughgoing cavity of a mold that is cooled so that before the metal reaches the lower end of the cavity the strand shell is strong enough to form a coherent shape that is pulled from the mold as a continuous strand. When steel strip is being formed the mold has two relatively long sides bridged at their ends by two relatively short ends. Granular additives are added at the top of the mold to form a protective and lubricating slag.
In order to make the faces of the strip as smooth as possible it has been suggested in European patent application 1,149,734 and in my earlier U.S. Pat. No. 4,721,151 to flare the mold cavity at an upper central region where the liquid metal is introduced into the mold, that is shape the cavity in this region so its flow cross section decreases downward. In the center of the top of the cavity the inside surfaces of the sides of the mold are cut back to achieve this effect. End regions of these inner surfaces are parallel to each other so that the tapering only actually exists at the upper region of the mold cavity in its center.
Such a system somewhat reduces surface imperfections in the steel strip produced, but still leaves a substantial number of flaws, particularly near the edges of the strip. The improvement is evidently caused by entraining a small amount of the slag from atop the melt down along the sides of the mold. The resultant lubrication substantially eliminates wavy marks on the faces of the steel strip thus produced.
Accordingly in my U.S. Pat. No. 4,834,167 I propose a system where the inner surfaces of the long sides are each formed by a planar lower portion wholly below the level, a central portion offset inward from the end walls and extending from the respective lower portion up above the level, a pair of planar intermediate portions coplanar with the respective lower portion, wholly below the level, and each extending between the respective central portion and a respective one of the end walls, and a pair of planar upper portions coplanar with each other, extending from the respective intermediate portions upward past the level, and flanking the respective central portion above the respective intermediate portions. The upper portions of each of the side walls diverge upward from and form a reflex angle of between 1' and 3' with a symmetry plane bisecting the side walls. The lower portions extend parallel to each other the full width of the mold between the end walls and the central portions flank the lower end of the tube pouring liquid steel into the mold and diverge upward. The end walls are of uniform horizontal width below the upper portions and are of a width increasing uniformly upward along the upper portions.
It has further been suggested in German patent document 3,907,351 of H. Grothe to form the recess of each upper central region as a curved pocket having an inwardly concave central portion flanked by two inwardly convex side portions. The radii of curvature of these portions increase in the downward flow direction through the mold until the curved portions become flat with an effective radius of infinity and merge with the planar inner face of the wide wall. This continuously deforms the incoming strand of molten steel so as to reduce friction between the strand shell and the mold in the critical upper region.
In U.S. Pat. No. 4,811,779 a mold is described whose enlarged upper region has an upper portion that extends parallel to the side walls to produce opposite deformation in the strand.